Most electrical systems are digital today and hence require analog-to-digital converters (ADCs) to interface to the outside world. The outside world can either be real world signals such as temperature, pressure, voice, etc., or modulated carriers transmitting information over some kind of medium (analog or digital communication). For all applications, energy efficiency is extremely important and more so for battery operated systems.
Delta sigma architectures are widely used for high resolution, low speed ADCs as well as for medium resolution, high speed ADCs. The more traditional implementation of a delta sigma modulator is in Discrete time Delta Sigma Modulator (DTDSM). DTDSMs are more robust to parameter variations as compared to continuous time delta sigma modulators (CTDSMs). This is because DTDSM parameters generally depend only on capacitor ratios which are well controlled while CTDSM parameters depend on RC time constants which are poorly controlled. However, DTDSMs typically charge and discharge capacitors in every cycle and hence are less power efficient as compared to CTDSM. Hence, CTDSMs are often preferred in energy saving applications such as battery operated equipment, body area networks etc.
A CTDSM is typically designed by starting from the DTDSM architecture and then converting the design to continuous time by using an impulse invariant transform. The CTDSM loop contains integrators, one or more quantizers and one or more feedback DACs (Digital to Analog converters). The quantizers and feedback DACs typically contribute to some delay in the loop which is termed as excess loop delay. DTDSMs do not have an issue with excess loop delay since all activity in a DTDSM occurs only on clock edges and any delay less than a clock cycle is not of significance. However in CTDSMs, the integrators are continuous time in nature i.e. they integrate all the time and hence excess loop delay modifies the transfer function of the loop thus degrading the stability and signal to noise ratio. CTDSMs are also more sensitive to clock jitter as compared to DTDSMs. In spite of all these issues, CTDSMs have gained in popularity due to their inherent energy efficiency. Thus, there is a need for techniques to mitigate the apparent drawbacks of CTDSMs.